Optical transmitter with sbs suppression

ABSTRACT

An optical transmitter and methods of generating an optical signal having SBS suppression are described. An optical transmitter having SBS suppression according to the present invention includes a signal generator that generates a SBS suppression signal. A laser generates a line width broadened optical signal having AM noise. A signal processor generates a modified SBS suppression signal from the SBS suppression signal. A modulator modulates the line width broadened optical signal having the AM noise with a payload modulation signal and with the modified SBS suppression signal to generate a payload modulated optical signal having SBS suppression and reduced AM noise.

BACKGROUND OF INVENTION

[0001] Optical fiber communication systems are lightwave systems thatemploy optical fibers to transmit information. Optical fibercommunications systems include optical transmitters, optical receivers,and transmission media that propagate information between the opticaltransmitters and the optical receivers. An optical transmitter for anoptical fiber communication system includes an optical source, such as asemiconductor laser, that generates an optical signal and an opticalmodulator that modulates the optical signal with data or voiceinformation. The modulated optical signal is transmitted through atransmission media, such as an optical fiber, to an optical receiver.The optical receiver detects the transmitted optical signal andprocesses the optical signal into an electronic waveform that containsthe data or voice information.

[0002] Optical fiber communication systems are now widely deployed.Recently, relatively new communications services, such as the Internet,high-speed data links, video services, wireless services and CATV, haveresulted in a dramatic increase in the need for higher information datarates. The aggregate data throughput rate of a communication system canbe increased either by increasing the bandwidth of an individual datachannel or by increasing the number of data channels.

[0003] In addition, many optical fiber communication systems today arebeing built to transmit data over long distances with high data rates.Moreover, such systems are currently being built to transmit data andvoice information over these longer distances without employingrepeaters in order to reduce the capital and operating costs associatedwith transmitting data. In order to achieve these higher data rates andlonger transmission distances, optical signals having relatively narrowline widths must be transmitted at relatively high intensity.

[0004] The noise detected at the receiver increases as the bandwidth ofan individual channel is increased. The amount of optical power in thetransmitted carrier signal must be increased to maintain a sufficientsignal-to-noise ratio at the receiver as the bandwidth of an individualchannel is increased. However, the propagation distance that can beachieved using a carrier signal with increased power in a narrow linewidth is severely limited by a physical effect known as stimulatedBrillouin scattering (SBS).

[0005] Stimulated Brillouin scattering is a stimulated scatteringprocess that converts a forward traveling optical wave into a backwardtraveling optical wave that is shifted in frequency relative to theforward traveling optical wave. Backward scattering occurs withinoptical fibers because of coupling between acoustic phonons created byvibrational excitation of acoustic modes in the optical fiber materialitself and by the incident photons of the optical signal.

[0006] The acoustic phonons and photons generate transient gratings thatproduce backward scattering and frequency shifting of the incidentoptical signals. The frequency shifting is typically between about10-100 MHz for commonly used optical communication fibers. StimulatedBrillouin scattering also causes multiple frequency shifts. In addition,SBS can permanently damage the optical fiber if the optical propagatingpower is sufficiently high.

[0007] The transmission quality of optical signals having relativelyhigh intensity and narrow line width can be improved by reducing theeffects of SBS. The increase in the transmission quality can allow dataand voice service providers to increase the optical signal power level,and therefore, increase the possible propagation distance of theircommunication links between repeaters. Consequently, reducing theeffects of SBS can reduce the cost per bit to transmit data and voiceinformation.

BRIEF DESCRIPTION OF DRAWINGS

[0008] This invention is described with particularity in the detaileddescription. The above and further advantages of this invention may bebetter understood by referring to the following description inconjunction with the accompanying drawings, in which like numeralsindicate like structural elements and features in various figures. Thedrawings are not necessarily to scale, emphasis instead being placedupon illustrating the principles of the invention.

[0009]FIG. 1 illustrates a block diagram of an optical transmitter thatsuppresses SBS by increasing the bandwidth of the optical signal and byreducing amplitude modulation noise according to the present invention.

[0010]FIG. 2 illustrates a block diagram of an embodiment of a SBSsuppression signal generator that generates a SBS suppression signalthat increases the line width of the optical signal according to thepresent invention.

[0011]FIG. 3 illustrates an embodiment of an integrated opticaltransmitter subassembly that can be used to generate an optical signalwith SBS suppression according to the present invention.

[0012]FIG. 4 illustrates another embodiment of an integrated opticaltransmitter subassembly that can be used to generate an optical signalwith SBS suppression according to the present invention.

[0013]FIG. 5 illustrates an embodiment of a fiber laser opticaltransmitter module that can be used to generate an optical signal withSBS suppression according to the present invention.

[0014]FIG. 6 illustrates a 500 kHz span of a frequency spectra generatedby a commercially available CATV receiver that received an opticalsignal that was generated by an optical transmitter according to thepresent invention without SBS suppression compensation.

[0015]FIG. 7 illustrates a 500 kHz span of a frequency spectra generatedby a commercially available CATV receiver that received an opticalsignal that was generated by an optical transmitter according to thepresent invention with SBS suppression compensation.

DETAILED DESCRIPTION

[0016] Optical fibers used for communications exhibit stimulatedBrillouin scattering (SBS) at optical signal power levels that are aslow as ˜1 mW in some optical fibers. The threshold optical power thatcauses SBS can be expressed by the following equation:

/_(th)≅21(α/G _(B))

[0017] where the parameter α represents absorption in the optical fiberand the parameter G_(B) represents the peak gain, which is approximately5×10⁻¹¹ m/W for narrow-bandwidth signals used for communications. Thepeak gain decreases as the incident optical signal bandwidth increases.For example, an optical fiber having an effective area of 50 μm², andhaving an absorption coefficient α≅0.2 dB/km, will exhibit a thresholdoptical power level which causes SBS that is approximately 2.4 mW for anoptical fiber length that is approximately 20 km.

[0018] Optical power levels that exceed the threshold optical power willcause the SBS to rapidly rise until the SBS limits the power that can betransmitted through the optical fiber. When the SBS limits the powerthat can be transmitted through the optical fiber, the power transmittedforward through the optical fiber will become nearly independent of thepower of the incident optical signal.

[0019] One method of suppressing SBS in optical fibers is to vary thephase angle of the optical signal with time in order to impose recurrentphase deviations that suppress SBS. Another method of suppressing SBS isto increase the effective optical source line width by using a carrierwaveform that has multiple frequencies. For example, a single opticalsource can be configured to generate two longitudinal optical modes withslightly different wavelengths that produce a beat frequency thatincreases the effective optical source line width.

[0020] Another method of suppressing SBS is to increase the line widthof the optical source by modulating the broadened optical signal with anoise source. The noise source can be frequency or phase modulated. Thebandwidth of the noise and/or the optical modulation index is controlledto provide a desired line width for the broadened optical signal. Forexample, one method of suppressing SBS generates white noise andextracts a component of the white noise having a predetermined frequencyband. The component of white noise is then superimposed on the biascurrent of the optical source to widen the line width of the opticalsource.

[0021] Another method of suppressing SBS is to tune the optical sourcewith a dither signal in order to increase the line width of the opticalsource. For example, one method of suppressing SBS uses a resonantcavity distributed feedback (DFB) laser with an external modulator tosuperimpose a dither signal onto the optical signal. This method isundesirable because it requires a relatively complex and expensivelaser.

[0022] These methods of increasing the line width of the optical sourcecan have the undesirable effect of generating residual signals or noisein the frequency band of interest or generating residual signals ornoise as intermodulation frequency products. These undesirable residualsignals or noise can be reduced or eliminated by applying a dithersignal to the laser bias drive signal that has a frequency which isoutside of the frequency band of interest. However, modern broadbandoptical communication systems typically require the use of a microwavefrequency dither signal in order to reduce these residual signals ornoise. Including such a microwave dither signal generator in thecommunication system is undesirable because it can significantlyincrease the overall cost of the system and can also cause undesirableelectromagnetic interference.

[0023] The methods and apparatus of the present invention suppress SBSin optical fiber transmission systems by increasing the line width ofthe optical signal and by reducing amplitude modulation (AM) noise.Suppression of stimulated Brillouin scattering is achieved by generatinga SBS suppression signal that can be a single coherent dither tone orany combination of signals that increases the optical signal line width,such as noise, pseudorandom noise sequences or other line widthincreasing techniques. The SBS suppression signal can have a bandwidththat is within or outside the data transmission bandwidth. Reducing theAM noise is achieved by modulating a cancellation signal.

[0024]FIG. 1 illustrates a block diagram of an optical transmitter 100that suppresses SBS by increasing optical signal bandwidth according tothe present invention and by reducing amplitude modulation noise. Theterm “optical transmitter” is defined herein to mean all devices(sources, modulators, amplifiers and processors) that prepare an opticalsignal for transmission through a transmission media. The transmitter100 includes a laser 102 that generates an optical signal at an output104. The laser 102 can be any type of laser that generates theappropriate optical signal and that is responsive to an electrical biascontrol signal.

[0025] The optical transmitter 100 includes a laser bias power supply106 having an output 108 that is electrically coupled to a bias input110 of the laser 102. The laser bias power supply 106 generates a biassignal at the output 108 that biases the laser 102 at the appropriateoperating point so that it emits the optical signal at the desiredwavelength and at the desired power level. In one embodiment, a detector(not shown) monitors the laser 102 and generates a feedback signal thatis used to change the bias signal that is generated by the laser biaspower supply 106 in order to control the wavelength and/or power of theoptical signal that is generated by the laser 102.

[0026] The optical transmitter 100 also includes a SBS suppressionsignal generator 112 that generates a SBS suppression signal at a first114 and a second output 116. The SBS suppression signal generator 112generates a signal that broadens the line width of the optical signalthat is generated by the laser 102. The SBS suppression signal generatoraccording to the present invention can generate any type of signal thatbroadens the line width of an optical signal. There are numerous typesof signals that are known to broaden the line width of an opticalsignal. For example, the SBS suppression signal can comprise a widerange of frequency spectra that can include random noise, pseudo randomnoise, and discrete tones.

[0027] The SBS suppression signal generator 112 can be any type ofsignal generator that generates a signal that broadens the line width ofthe optical signal that is generated by the laser 102. For example, theSBS suppression signal generator 112 can include a random noise source,a deterministic noise source, such as a pseudorandom sequence source, adither signal generator, or a coherent signal sources. In oneembodiment, the SBS suppression signal generator 112 includes a filter(not shown) that limits the bandwidth of the SBS suppression signal. Inone embodiment, the SBS suppression signal generator 112 includes amodulator (not shown) that generates a modulation signal that broadensthe line width of the optical signal. An exemplarily embodiment of theSBS suppression signal generator 112 is described in connection withFIG. 2.

[0028] The first output 114 of the SBS suppression signal generator 112is electrically connected to an input 118 of the laser bias power supply106. The transmitter 100 also includes an optical modulator 120 that isoptically coupled to the output 104 of the laser 102. The opticalmodulator 120 modulates the optical signal that is generated by thelaser 102. The present invention is described with embodiments that useexternal modulation. However, the present invention can be practicedwith any type of modulation.

[0029] The optical modulator 120 can be any type of optical modulatorthat is responsive to an electrical modulation signal. In oneembodiment, the optical modulator 120 is an electro-absorptionmodulator. In another embodiment, the optical modulator 120 is a MachZehnder-type interferometric modulator. In still another embodiment, theoptical modulator 120 is an optical gain modulator, such as asemiconductor optical amplifier (SOA) or other gain modulating device.

[0030] A modulation signal generator 122 generates a payload modulationsignal at an output 124. In some embodiments, the modulation signalgenerator 122 also generates a DC bias offset voltage that biases theoptical modulator 120 at the appropriate operating point. In theembodiment illustrated in FIG. 1, an output 126 of a separate DC biasvoltage power supply 128 is electrically coupled to a bias input 130 ofthe optical modulator 120.

[0031] The optical transmitter 100 also includes a signal processor 132that has an input 134 that is electrically coupled to the output 116 ofthe SBS suppression signal generator 112. The signal processor 132processes the SBS suppression signal that is generated by the SBSsuppression signal generator 112 and produces a modified SBS suppressionsignal at an output 136. The modified SBS suppression signal is a signalthat is used to reduce or suppress amplitude modulation (AM) noisecaused by the SBS suppression signal that is used to spread the linewidth of the optical signal generated by the laser 102. The modified SBSsuppression signal, however, maintains the desired line spreading in theoptical signal. The term “AM noise” is defined herein to meanundesirable amplitude modulation or residual signals.

[0032] A summation circuit 140 is used to combine the payload modulationsignal that is generated by the modulation signal generator 122 with themodified SBS suppression signal that is generated by the signalprocessor 132. In the embodiment shown in FIG. 1, the output 124 of themodulation signal generator 122 is electrically coupled to a first input142 of the summation circuit 140. The output 136 of the signal processor132 is electrically coupled to a second input 144 of the summationcircuit 140. An output 146 of the summation circuit 140 is electricallycoupled to an input 148 of the modulator 120. In some embodiments (notshown), the processed SBS suppression signal and the payload modulationsignal are electrically coupled to different inputs (not shown) of theoptical modulator 120. In other embodiments, the processed SBSsuppression signal and the payload modulation signal are electricallycoupled to separate gain or loss elements as shown in FIG. 3.

[0033] In operation, the laser bias power supply 106 generates a laserbias signal that is sufficient to cause the laser 102 to generate theoptical signal with the desired wavelength and power level. The SBSsuppression signal generator 112 generates a SBS suppression signal thatincreases the bandwidth of the optical signal that is generated by thelaser 102. For example, the SBS suppression signal can be a bandwidthlimited noise signal, a discrete tone, or a combination of noise signalsand discrete tones. In one embodiment, a phase shifter 138 shifts thephase of the SBS suppression signal.

[0034] The laser bias signal is combined with the SBS suppression signaland then the combined signal is coupled to the input 110 of the laser102. The combined laser bias signal and SBS suppression signal drivesthe laser 102 to generate an optical signal having increased bandwidthat the output 104.

[0035] The signal processor 132 processes the SBS suppression signal andgenerates a modified SBS suppression signal at the output 136. In oneembodiment, the phase shifter 138 applies a phase shift to the SBSsuppression signal and the signal processor 132 processes thephase-shifted SBS suppression signal and generates the modified SBSsuppression signal at the output 136. In one embodiment, the signalprocessor 132 shifts the phase of the SBS suppression signal. Phaseshifting can be used to achieve vector cancellation that compensates foran imperfect match of the phase response between the optical signal thatis generated by the laser 102 and the optical signal modulated by themodulator 120.

[0036] In one embodiment, the modified SBS suppression signal has asignal bandwidth that is within the frequency range of the payloadmodulation signal. In another embodiment, the modified SBS suppressionsignal has a signal bandwidth that is outside the payload modulationsignal bandwidth.

[0037] In one embodiment, the modified SBS suppression signal includes acancellation signal that reduces or substantially cancels AM noise inthe optical modulation signal that is caused by the SBS suppressionsignal. For example, in one embodiment, the modified SBS suppressionsignal is a complementary SBS suppression signal. In this embodiment,the signal processor 132 generates a modified SBS suppression signalthat includes an inverted replica of the SBS suppression signal. Theterm “replica” is defined herein to mean a substantially exact orapproximate copy of the signal that causes approximate vectorcancellation of the SBS suppression signal.

[0038] In another embodiment, the signal processor 132 generates amodified SBS suppression signal that includes an inverted replica of theSBS suppression signal that is phase shifted relative to the SBSsuppression signal that is used to drive the laser 102. In yet anotherembodiment, the signal processor 132 generates a modified SBSsuppression signal that includes an inverted replica of the SBSsuppression signal that is scaled in amplitude relative to the SBSsuppression signal that is used to drive the laser 102.

[0039] In other embodiments, the signal processor 132 generates amodified SBS suppression signal that includes a signal that is amathematical transform of the SBS suppression signal. In someembodiments, the mathematical transform is chosen to cause vectorcancellation of the SBS suppression signal. In other embodiments, themathematical transform is chosen to produce a pre-distortion thatcompensates for modulation non-linearities caused by the laser 102and/or the modulator 120. In yet other embodiments, the signal processor132 generates a modified SBS suppression signal that includes a signalhaving a harmonic of the SBS suppression signal.

[0040] The summation circuit 140 combines the payload modulation signalwith the modified SBS suppression signal and applies the combined signalto the input 148 of the modulator 120. The optical modulator 120modulates the optical signal that is generated by the laser 102 with thecombined signal. The AM noise portion of the SBS suppression signal isreduced or substantially canceled in the resulting modulated opticalsignal.

[0041] Experimental results have been obtained by using a SBSsuppression signal generator 112 that includes a thermally generatednoise signal. These experimental results indicate that greater than 10dB of AM noise suppression is achievable for noise that is bandwidthlimited to 20 kHz. Experimental results have also been obtained by usinga SBS suppression signal generator 112 that includes a discreteelectrical tone generator. These experimental results indicate that 20dB of AM noise suppression is achievable when injecting a single 9 kHztone. The experimental results show that the AM noise suppression issufficient to reduce the AM noise detected at an optical receiver tonegligible levels for the applied signal power levels that are requiredto produce the desired frequency spreading.

[0042]FIG. 2 illustrates a block diagram of an embodiment of a SBSsuppression signal generator 200 that generates a SBS suppression signalthat increases the line width of the optical signal according to thepresent invention. The SBS suppression signal generator 200 includes anoise source 202 that generates a noise signal at an output 204. In theembodiment shown, the noise source 202 is a Zener diode noise source.However, numerous other noise sources can be used. In other embodiments,a signal generator (not shown) is used to generate a single ormulti-frequency waveform that is used to produce the SBS suppressionsignal. In these embodiments, the SBS suppression signal generator 200can produce waveforms, such as sinusoidal, ramp, or other waveforms thatare used to generate the SBS suppression signal.

[0043] The output 204 of the noise source 202 is capacitively coupled toan input 206 of a voltage amplifier 208. The voltage amplifier 208amplifies the noise signal to a signal level that is appropriate for SBSsuppression. An output 210 of the voltage amplifier 208 is electricallycoupled to an input 212 of a low-pass filter 214. The low-pass filter214 passes a portion of the noise signal that has frequency componentsbelow a certain cut-off frequency of the low-pass filter 214. An output216 of the low-pass filter 214 is electrically coupled to an input 218of a high-pass filter 220. The high-pass filter 220 passes a portion ofthe low-pass filtered noise signal that has frequency components above acut-off frequency of the high-pass filter 220.

[0044] Experimental data are shown in FIGS. 6 and 7 for an embodiment ofthe SBS suppression signal generator 112 (FIG. 1) where the low passfilter 214 comprises a 50 kHz fourth-order Butterworth filter and thehigh-pass filter 220 comprises a second-order Butterworth filter that istuned to 1 kHz.

[0045]FIG. 3 shows an embodiment of an integrated optical transmittersubassembly 300 that can be used to generate an optical signal with SBSsuppression according to the present invention. In one embodiment, thetransmitter subassembly 300 is an opto-electronic integrated circuit. Inanother embodiment, the transmitter subassembly 300 comprises discretecomponents.

[0046] Referring to FIG. 1 and FIG. 3, the transmitter subassembly 300includes a laser diode 302 that generates an optical signal at an output304 with the appropriate wavelength and power level. An input 308 of thelaser diode 302 is electrically coupled to the output 108 of the laserbias power supply 106 which is driven by the output 114 of the SBSsuppression signal generator 112.

[0047] In one embodiment, the transmitter subassembly 300 includes aback facet diode detector 306 that generates a feedback signal at anoutput 307. The feedback signal can be used to change the bias signalthat is generated by the laser bias power supply 106 in order to controlthe wavelength and/or power of the optical signal that is generated bythe laser 302.

[0048] The transmitter subassembly 300 also includes a SOA diode 310,such as a Fabry Perot or traveling wave semiconductor amplifier. The SOAdiode 310 is optically coupled to the output 304 of the laser diode 302.An input 312 of the SOA diode 310 is electrically coupled to the output136 of the signal processor 132 and the output of an SOA bias power (notshown in FIG. 1) that is used to bias the SOA diode 310 at the desiredoperating point. The SOA diode 310 is an in-line optical amplifier thatgenerates an optical signal at an output 314 that is gain modulated bythe SBS suppression signal.

[0049] The transmitter subassembly 300 also includes anelectro-absorption (EA) diode 316 that is optically coupled to theoutput 314 of the SOA diode 310. An input 318 of the EA diode 316 iselectrically coupled to the output 124 of the modulation signalgenerator 122 and to the output 126 of the modulation bias power supply128.

[0050] The operation of the transmitter subassembly 300 is similar tothe operation of the optical transmitter 100 that was described inconnection with FIG. 1. Referring to FIG. 1 and FIG. 3, the laser biaspower supply 106 generates a laser bias signal that is sufficient tocause the laser diode 302 to generate the optical signal with thedesired wavelength and power level. The SBS suppression signal generator112 generates a SBS suppression signal that increases the bandwidth ofthe optical signal that is generated by the laser diode 302. The laserbias signal and the SBS suppression signal are applied to the input 308of the laser diode 302. The laser diode 302 generates an optical signalhaving increased bandwidth at the output 304.

[0051] The signal processor 132 processes the SBS suppression signal andgenerates a modified SBS suppression signal at the output 136. In oneembodiment, the modified SBS suppression signal includes a cancellationsignal that reduces or substantially cancels undesirable AM noise in theoptical modulation signal that is caused by the SBS suppression signalas described in connection with FIG. 1. The output 136 of the signalprocessor 132 is applied to the input 312 of the SOA diode 310. A SOAbias power supply (not shown) generates a bias signal that biases theSOA diode 310 at an operating point that produces the desired gainmodulation. The bias signal is applied to the input 312 of the SOA diode310. The SOA diode 310 generates an amplified optical signal at theoutput 314 that is gain modulated by the modified SBS suppressionsignal.

[0052] The EA diode 316 is biased by the modulation bias signal that isgenerated at the output 126 of the modulation bias power supply 128. TheEA diode 316 modulates the amplified optical signal that is gainmodulated by the modified SBS suppression signal with the modulationpayload signal that is generated by the modulation signal generator 122.The EA diode 316 generates a payload modulation signal at an output 320.

[0053] The payload modulation signal has SBS suppression resulting fromthe increased line width produced by the SBS suppression signal. Thepayload modulation signal also has reduced AM noise because the opticalsignal is gain modulated by the modified SBS suppression signal. Inaddition, the payload modulation signal has a relatively highsignal-to-noise ratio because the full modulation depth of the EA diode316 is available for the payload.

[0054]FIG. 4 illustrates another embodiment of an integrated opticaltransmitter subassembly 350 that can be used to generate an opticalsignal with SBS suppression according to the present invention. Theintegrated optical transmitter subassembly 350 of FIG. 4 can be lessexpensive to manufacture compared with the integrated opticaltransmitter subassembly 300 of FIG. 3. The transmitter subassembly 350is similar to the transmitter subassembly 300 that was described inconnection with FIG. 3. However, the optical transmitter subassembly 350does not include the SOA diode 310 shown in FIG. 3. Instead, the EAdiode 316 of the transmitter subassembly 350 is electrically coupled tothe output 136 of the signal processor 132 (FIG. 1), the output 124 ofthe modulation signal generator 122 (FIG. 1), and to the output 126 ofthe modulation bias power supply 128 (FIG. 1).

[0055] The operation of the transmitter subassembly 350 is similar tothe operation of the transmitter subassembly 300 that was described inconnection with FIG. 3. Referring to FIG. 1, FIG. 3 and FIG. 4, thelaser bias power supply 106 generates a laser bias signal that issufficient to cause the laser diode 302 to generate the optical signalwith the desired wavelength and power level. The SBS suppression signalgenerator 112 generates a SBS suppression signal that increases thebandwidth of the optical signal generated by the laser diode 302. Thelaser bias signal and the SBS suppression signal are applied to theinput 308 of the laser diode 302. The laser diode 302 generates anoptical signal having increased bandwidth at the output 304.

[0056] The signal processor 132 processes the SBS suppression signal andgenerates a modified SBS suppression signal at the output 136. In oneembodiment, the modified SBS suppression signal includes a cancellationsignal that reduces or substantially cancels amplitude modulation noisein the optical modulation signal that is caused by SBS suppressionsignal as described herein. The output of the signal processor 132 isapplied to the input 318 of the EA diode 316.

[0057] The EA diode 316 is biased by the signal that is generated at theoutput 126 of the modulation bias power supply 128. The EA diode 316modulates the optical signal that is generated by the laser diode 302with the modified SBS suppression signal that is generated by the signalprocessor 132 and with the modulation payload signal that is generatedby the modulation signal generator 122. The EA diode 316 generates apayload modulation signal at an output 320.

[0058] The payload modulation signal has SBS suppression resulting fromthe increased line width produced by the SBS suppression signal. Thepayload modulation signal also has reduced AM noise because the opticalsignal is modulated by the modified SBS suppression signal. However, thepayload modulation signal has a somewhat lower signal-to-noise ratiocompared with the payload modulation signal that is generated by thetransmitter subassembly 300 of FIG. 3 because some of the modulationdepth of the EA diode 316 is used to modulate the modified SBSsuppression signal.

[0059]FIG. 5 illustrates an exemplary embodiment of a fiber laseroptical transmitter subassembly 400 that can be used to generate anoptical signal with SBS suppression according to the present invention.The transmitter subassembly 400 is similar to the transmittersubassembly 300 that was described in connection with FIG. 3. However,the transmitter subassembly 400 includes a fiber laser 402. There arenumerous types of fiber lasers that are well known in the art. Althougha Fabry Perot-type fiber laser is shown in FIG. 5, any type of fiberlaser can be used with the transmitter subassembly 400 of the presentinvention.

[0060] Referring to FIG. 1 and FIG. 5, the fiber laser 402 includes amodulation input 404 that is electrically coupled to the output 114 ofthe SBS suppression signal generator 112. A laser bias power supply (notshown) generates a laser bias signal that is used to bias the fiberlaser 402 at an operating point that causes the fiber laser 402 togenerate an optical signal having the desired wavelength and powerlevel.

[0061] The transmitter module 400 also includes a discrete SOA 406, suchas a Fabry Perot or traveling wave semiconductor amplifier. An opticalinput 408 of the SOA 406 is optically coupled to the output of the fiberlaser 402. An electrical input 410 of the SOA 406 is electricallycoupled to the output 136 of the signal processor 132 and the output ofan SOA bias power (not shown in FIG. 1) that is used to bias the SOA 406at the desired operating point. The SOA 406 is an in-line opticalamplifier that generates an optical signal at an output 412 that is gainmodulated by the SBS suppression signal.

[0062] The transmitter module 400 also includes an electro-opticmodulator 414. Any type of electro-optic modulator can be used in thetransmitter module 400. For example, in one embodiment, the modulator414 is an electro-absorption modulator. In another embodiment, themodulator 414 is a Mach-Zehnder interferometric modulator. The output412 of the SOA 406 is optically coupled to an input 416 of the opticalmodulator 414. An input 418 of the modulator 414 is electrically coupledto the output 124 of the modulation signal generator 122 and to theoutput 126 of the modulation bias power supply 128.

[0063] The operation of the transmitter module 400 is similar to theoperation of the transmitter subassembly 300 that was described inconnection with FIG. 3. Referring to FIG. 1 and FIG. 5, the fiber laser402 generates an optical signal with the desired wavelength and powerlevel. The SBS suppression signal generator 112 generates a SBSsuppression signal. The SBS suppression signal is then applied to themodulation input 404 of the fiber laser 402. The SBS suppression signalincreases the bandwidth of the optical signal.

[0064] The signal processor 132 processes the SBS suppression signal andgenerates a modified SBS suppression signal at the output 136. In oneembodiment, the modified SBS suppression signal includes a cancellationsignal that reduces or substantially cancels AM noise in the opticalmodulation signal that is caused by SBS suppression signal as describedherein.

[0065] The output 136 of the signal processor 132 is applied to theinput 410 of the SOA 406. A SOA bias power supply (not shown) generatesa bias signal that biases the SOA 406 at an operating point thatproduces the desired gain modulation. The bias signal is applied to theinput 410 of the SOA 406. The SOA 406 generates an amplified opticalsignal at the output 412 that is gain modulated by the modified SBSsuppression signal.

[0066] The modulator 414 is biased by the signal that is generated atthe output 126 of the modulation bias power supply 128. The modulator414 modulates the amplified optical signal that is gain modulated by themodified SBS suppression signal with the modulation payload signal thatis generated by the modulation signal generator 122. The modulator 414generates a payload modulation signal at an output 420. The payloadmodulation signal has SBS suppression resulting from the increased linewidth produced by the SBS suppression signal. The payload modulationsignal also has reduced AM noise because the optical signal is modulatedby the modified SBS suppression signal.

[0067]FIG. 6 illustrates a 500 kHz span of a frequency spectra 450generated by a commercially available CATV receiver that received anoptical signal that was generated by an optical transmitter according tothe present invention without SBS suppression compensation. Referring toFIG. 1 and FIG. 3, the SBS suppression signal generator 112 generatedthe SBS suppression signal from amplified filtered noise produced by a50 kHz noise source. The SBS suppression signal was applied to the laser302. The modulation signal generator 122 generated a single 300 MHzcarrier tone modulation signal and applied the modulation signal to theEA diode 316. However, the signal processor 132 was not activated anddid not apply a modified SBS suppression signal to the SOA 310.Therefore, the SBS suppression was not active in the transmitter.

[0068] The optical power of the received signal was about −1 dBm. Thefrequency spectra 450 illustrated in FIG. 6 shows AM noise as amplitudemodulated sidebands. Specifically, the 50 kHz noise signal is clearlyvisible more than 10 dB above the noise floor on either side of thecarrier for the illustrated 500 kHz span. FIG. 7 illustrates the abilityto suppress the AM noise caused by the SBS suppression signal in theoptical signal.

[0069]FIG. 7 illustrates a 500 kHz span of a frequency spectra 500generated by a commercially available CATV receiver that received anoptical signal that was generated by an optical transmitter according tothe present invention with SBS suppression compensation. The opticaltransmitter that generated the optical signal was identical to thetransmitter that generated the optical signal described in connectionwith FIG. 6, but the signal processor was active in the transmitter.Therefore, the SBS suppression compensation was active in thetransmitter.

[0070] Referring to FIG. 1 and FIG. 3, the SBS suppression signalgenerator 112 generated the SBS suppression signal from amplifiedfiltered noise produced by a 50 kHz noise source with the same level ofnoise used to generate the SBS suppression signal that was described inconnection with FIG. 6. The SBS suppression signal was applied to thelaser 302. The modulation signal generator 122 generated a single 300MHz carrier tone modulation signal and applied the modulation signal tothe EA diode 316. The signal processor 132 was activated so as to applya modified SBS suppression signal to the SOA 310. Therefore, the SBSsuppression was activated in the transmitter.

[0071] The optical power of the received signal was about −1 dBm. Thefrequency spectra 500 illustrated in FIG. 7 illustrates the same 300 MHzcarrier with the same level of noise. However, applying the modified SBSsuppression signal to the SOA 310 substantially eliminated the AM noise.

[0072] Equivalents

[0073] While the invention has been particularly shown and describedwith reference to specific preferred embodiments, it should beunderstood by those skilled in the art that various changes in form anddetail may be made therein without departing from the spirit and scopeof the invention as defined herein.

What is claimed is:
 1. An optical transmitter having SBS suppression,the optical transmitter comprising: a signal generator that generates aSBS suppression signal at an output; a laser having an electrical inputthat is electrically coupled to the output of the signal generator, thelaser generating an optical signal at an output having a line width thatis broadened by the SBS suppression signal and having AM noise; a signalprocessor having an electrical input that is electrically coupled to theoutput of the signal generator, the signal processor generating amodified SBS suppression signal at an output; and a modulator that isoptically coupled to the output of the laser and that is electricallycoupled to the output of the signal processor, the modulator modulatingthe optical signal with a payload modulation signal and with themodified SBS suppression signal, thereby generating a payload modulatedoptical signal having SBS suppression, wherein the modified SBSsuppression signal reduces the AM noise in the payload modulated opticalsignal.
 2. The transmitter of claim 1 wherein the AM noise in thepayload modulated optical signal is substantially eliminated.
 3. Thetransmitter of claim 1 wherein the signal generator comprises a noisesource that generates a noise signal that broadens the line width of theoptical signal.
 4. The transmitter of claim 1 wherein the signalgenerator comprises a pseudorandom sequence source that generates apseudorandom sequence that broadens the line width of the opticalsignal.
 5. The transmitter of claim 1 wherein the signal generatorcomprises a coherent signal source that generates a coherent signal thatbroadens the line width of the optical signal.
 6. The transmitter ofclaim 1 wherein the signal generator comprises a modulator thatgenerates a modulation signal that broadens the line width of theoptical signal.
 7. The transmitter of claim 1 wherein the signalgenerator comprises a filter that limits the bandwidth of the SBSsuppression signal.
 8. The transmitter of claim 1 wherein the lasercomprises a diode laser that generates the optical signal.
 9. Thetransmitter of claim 1 wherein the laser comprises a fiber laser thatgenerates the optical signal.
 10. The transmitter of claim 1 furthercomprising a laser bias power supply that is electrically coupled to theelectrical input of the laser, the laser bias power supply generating abias current that drives the laser.
 11. The transmitter of claim 1wherein the signal processor shifts a phase of the SBS suppressionsignal.
 12. The transmitter of claim 1 wherein the signal processorchanges an amplitude of the SBS suppression signal.
 13. The transmitterof claim 1 wherein the modified SBS suppression signal comprises aninverted replica of the SBS suppression signal.
 14. The transmitter ofclaim 13 wherein the inverted replica of the SBS suppression signal isshifted in phase relative to the SBS suppression signal.
 15. Thetransmitter of claim 13 wherein the inverted replica of the SBSsuppression signal is scaled in amplitude relative to the SBSsuppression signal.
 16. The transmitter of claim 1 wherein the modulatorcomprises an amplifier that gain modulates the optical signal.
 17. Thetransmitter of claim 16 wherein the amplifier comprises a semiconductoroptical amplifier.
 18. The transmitter of claim 1 wherein the modulatorcomprises an electro-absorption modulator.
 19. The transmitter of claim1 wherein the modulator comprises a Mach-Zehnder interferometricmodulator.
 20. The transmitter of claim 1 further comprising amodulation signal generator having an output that is electricallycoupled to a modulation input of the modulator, the modulation signalgenerator generating the payload modulation signal.
 21. The transmitterof claim 1 wherein the signal generator generates a SBS suppressionsignal that has a signal bandwidth that is within a signal bandwidth ofthe payload modulation signal.
 22. The transmitter of claim 1 whereinthe signal generator generates a SBS suppression signal that has asignal bandwidth that is outside a signal bandwidth of the payloadmodulation signal.
 23. A method of generating a modulated optical signalhaving SBS suppression, the method comprising: generating a SBSsuppression signal; modulating an optical signal with the SBSsuppression signal, thereby generating a line width broadened opticalsignal having AM noise; generating a modified SBS suppression signalfrom the SBS suppression signal; and modulating the line width broadenedoptical signal with a payload modulation signal and with the modifiedSBS suppression signal, thereby generating a payload modulated opticalsignal having SBS suppression, the modified SBS suppression signalreducing the AM noise in the payload modulated optical signal having SBSsuppression.
 24. The method of claim 23 wherein the AM noise in thepayload modulated optical signal having SBS suppression is substantiallyeliminated.
 25. The method of claim 23 wherein the generating the SBSsuppression signal comprises generating a noise signal that broadens theline width of the optical signal.
 26. The method of claim 23 wherein thegenerating the SBS suppression signal comprises generating a bandwidthlimited noise signal that broadens the line width of the optical signal.27. The method of claim 23 wherein the generating the SBS suppressionsignal comprises generating a pseudorandom sequence that broadens theline width of the optical signal.
 28. The method of claim 23 wherein thegenerating the SBS suppression signal comprises generating at least onecoherent signal that broadens the line width of the optical signal. 29.The method of claim 23 wherein the generating the SBS suppression signalcomprises modulating a signal that broadens the line width of theoptical signal.
 30. The method of claim 23 wherein the generating theSBS suppression signal comprises generating a SBS suppression signalthat has a signal bandwidth that is within the payload modulation signalbandwidth.
 31. The method of claim 23 wherein the generating the SBSsuppression signal comprises generating a SBS suppression signal thathas a signal bandwidth that is outside the payload bandwidth.
 32. Themethod of claim 23 wherein the generating the modified SBS suppressionsignal comprises shifting a phase of the SBS suppression signal.
 33. Themethod of claim 23 wherein the generating the modified SBS suppressionsignal comprises inverting the SBS suppression signal.
 34. The method ofclaim 23 wherein the generating the modified SBS suppression signalcomprises changing an amplitude of the SBS suppression signal.
 35. Amethod of reducing AM noise in a line width broadened modulated opticalsignal, the method comprising: generating a line width broadeningsignal; modulating an optical signal with the line width broadeningsignal, thereby generating a line width broadened optical signal havingAM noise; generating a modified line width broadening signal from theline width broadening signal; and modulating the line width broadenedoptical signal with the modified line width broadening signal, therebygenerating a line width broadened modulated optical signal, the modifiedline width broadening signal reducing the AM noise in the line widthbroadened modulated optical signal.
 36. The method of claim 35 whereinthe AM noise in the modulated line width broadened signal issubstantially eliminated.
 37. The method of claim 35 wherein thegenerating the modified line width broadening signal comprises invertingthe line width broadening signal.
 38. The method of claim 35 wherein thegenerating the modified line width broadening signal comprises shiftinga phase of the line width broadening signal.
 39. An optical transmittercomprising: means for generating a SBS suppression signal; means formodulating an optical signal with the SBS suppression signal, therebygenerating a line width broadened optical signal having AM noise; meansfor generating a modified SBS suppression signal from the SBSsuppression signal; and means for modulating the line width broadenedoptical signal with a payload modulation signal and with the modifiedSBS suppression signal, thereby generating a payload modulated opticalsignal having SBS suppression, the modified SBS suppression signalreducing the AM noise in the payload modulated optical signal having SBSsuppression.